1. Field of the Invention
The present invention relates to a method of purification of salt solutions to be subjected to electrolysis, and more particularly to a method of removing iodine and silica ions from the salt solutions, which ions adversely affect electrolysis, using zirconium hydroxide.
2. Description of the Related Art
It has been known that when a salt solution (brine) is subjected to electrolysis by using an ion exchange membrane method, iodine ions of around 1 ppm (weight) and silica ion of 10 to 20 ppm (weight) contained in the raw salt solution adversely affect electrolysis performance, and in order to overcome such a problem, various methods have been used to remove iodine and silica ions contained in the salt solutions to be electrolyzed. For example, an ion exchange method as disclosed in Japanese Laid-open Patent Application No. Hei-7-237919, an activated carbon adsorption method as disclosed in Japanese Post-examined Patent Application No. Hei-7-91666, a sedimentation method as disclosed in Japanese Post-examined Patent Application No. Hei-6-88777, etc. have been proposed as methods of removing iodine ions from the salt solutions.
For silica ion removal from salt solutions, there have been proposed the magnesium ion addition method disclosed in Japanese Post-examined patent application No. Sho-55-3290, etc., the primary refined sedimentary slurry circulation method disclosed in Japanese Post-examined patent application No. Sho-59-43556, etc. and the chelating resin adsorption method disclosed in Japanese Laid-open patent application No. Sho-60-125388, etc.
However, it has been found that the above conventional methods have the following disadvantages.
According to the ion exchange method as disclosed in Japanese Laid-open Patent Application No. Hei-7-237919, iodine ions are oxidized into iodine complex ions, and then the iodine complex ions thus obtained are removed through ion exchange using an anion exchange resin. However, it is difficult to surely control the oxidation of iodine ions to iodine complex ions. For example, iodate ions are also produced as one type of oxidized products obtained by oxidizing iodine ions. However, iodate ions cannot be adsorbed by the anion exchange resin. Therefore, when iodate ions are produced, some of iodine ions remain in the salt solution even when the salt solution is passed through the anion exchange resin. Accordingly, this method has a low efficiency in removal of iodine ions, and it is difficult to reduce the concentration of the iodine ions down to 0.2 mg/l or less, which is the maximum for the ion exchange membrane electrolysis process.
In the activated carbon adsorption method as disclosed in Japanese Post-examined Patent Application No. Hei-7-91666, it is difficult to surely control the oxidation of iodine ions to iodine complex ions. In addition, the iodine complex ions thus produced are reduced into iodide ions by activated carbon, and the adsorption efficiently is thereby reduced.
According to the sedimentation method as disclosed in Japanese Post-examined Patent Application No. Hei-6-88777, iodine ions are oxidized into periodate ions to form and deposit periodate which is only sparingly soluble, thereby removing the iodine ions. However, this method has also a low removal efficiency for iodine ions.
Among the above references disclosing silica ion removal, the magnesium ion addition method disclosed in Japanese Post-examined patent application No. Sho-55-3290 involves silica ion adsorption by magnesium hydroxide ion produced by adding magnesium chloride solution to the raw salt solution, but needs further improvement because of the high cost of disposal of the salt solution mud generated by the addition of magnesium hydroxide.
The primary refined sedimentary slurry circulation method disclosed in Japanese Post-examined patent application No. Sho-59-43556 forms silica salts from the silica ions by circulation of a portion of the primary refined sedimentary slurry of calcium carbonate or magnesium hydroxide, etc., settled in a sedimentation vessel. However, this method cannot reduce silica ion concentration to less than 2 ppm (weight) as SiO.sub.2.
The chelating resin adsorption method disclosed in Japanese Laid-open patent application No. Sho-60-125388 treats salt solutions with a strong cationic micro-porous type chelating resin under a slightly acidic condition. But this method has a lower silica ion removal efficiency than the above two methods.